When the ambient relative humidity drops to 50% or below, the human body accumulates a large electrical charge. For example, when the relative humidity is 25%, a person walking across a carpet may acquire a potential of 10 kV or higher. If a person carrying such a charge comes into contact with, or passes very close to, an item of electrical equipment, such as a telephone or radio, the charge will be discharged to it. Most small electrical equipment have housings made of a dielectric material such as plastics; and the housings contain holes for various purposes, such as to expose switches. If a discharge occurs between a charged object and such a piece of equipment, it will occur to or from a conductive portion, such as a printed-circuit board within the housing, through one of the holes. A static discharge to one of the components or to the circuit trace of the printed-circuit board may cause erroneous operation of the equipment or even failure of one of the components.
A common remedial measure, at least in areas of the housing that have a high concentration of holes, is the insertion of a conductive sheet between the printed-circuit board and the housing. The conductive sheet, which has holes for the keys corresponding to the holes in the housing, is fastened, usually at one end, to a suitable ground. Since the conductive sheet is the closest of all of the conductors within the housing to the holes, static discharges through the holes terminate at the conductive sheet rather than at the printed circuit board.
Since the sheet has a comparatively large area so that it surrounds a large number of holes in the housing, it must be made very thin to save both weight and space. Consequently, it is apt to flex and to short parts of the circuit trace on the printed-circuit board. For this reason, an insulation pad is usually inserted between the conductive sheet and the printed-circuit board. The insulation pad sometimes has holes for the leads of keys to be extended to the printed-circuit board.
When isolated holes have to be formed in the housing of a piece of electrical equipment, several possibilities exist for preventing static discharge damage to printed-circuit boards mounted within the housing adjacent to the holes. First, a separate conductive sheet, similar to that used for concentrations of holes, may be grounded and inserted between the housing and the printed-circuit board. In this case, however, there is a difficulty in fabricating the separate conductive sheets so that they may be held in place within the housing. The other possibility is to use a single conductive sheet surrounding all of the holes in the housing. Although a single, large sheet may easily be fixed in place in the housing, it is wasteful of the small amount of available space.